The need for communication bandwidth capacity has increased dramatically in the last two decades and continues on an exponential growth path. To fill this need communications companies have invested large sums into developing infrastructures to transmit information. One of the various methods of transmitting large quantities of information that has experienced much growth in the last decade utilizes optical fibers and transmits information in the form of modulated optical signals through these fibers. A communication system using optical fiber use transmitters at one end that typically convert electrical signals into optical signals that are transmitted through the fiber and receivers that convert optical signals into electrical signals at the other end of the fiber carrying the optical signal.
At various points throughout a fiber optic communication system, it is necessary to connect two fibers to each other where an optical signal being carried from one fiber is diverted into the other fiber. It is not atypical for an optical signal to travel long distances of up to several hundreds of kilometers on a single fiber. Even though optical signals can travel long distances without major deterioration in the quality of the signal, as a result of chromatic dispersion or other losses, it is necessary to use regenerators or optical amplifiers to restore optical signal quality. The use of such optical amplifiers, regenerators, etc., necessitates interconnections of two or more fibers.
It is important to have accurate alignment of optical signals at various fiber interconnection points. While many fiber to fiber connections can be accomplished with fiber fusion, a significant number of fiber interconnections require routing through free space. It is well known that precise alignment of fibers and other components at interconnection of fibers is a very time consuming and costly task. For example, as currently practiced, packaging of individual optoelectronic components with optic fibers accounts for 40–50 percent of the total product cost of such assembly. The problem is the need to align optoelectronic components and optical fibers with submicron (<1×10−6 m) precision. This submicron or nanometer precision is even more critical for interconnections that target high coupling efficiency.
Apart from the alignment problem discussed above, it is often necessary to adjust the coupling efficiency of an interconnection and various devices are used at the fiber interconnections to achieve the desired level of coupling efficiency between fibers or between a fiber and an opto-electronic device, such as a laser diode, etc. One of the commonly used devices to achieve desired coupling efficiency at interconnection points between two fibers or between a fiber and an opto-electronic device is a variable optical attenuator (VOA). A VOA is a device capable of producing a desired reduction in the strength of an optical signal transmitted through an optical fiber. In modem day communication systems, VOAs are key components of optical networks, including local and long distance telephone networks. High performance telecommunication systems rely on VOAs to perform power equalization after a variety of network functions including filtering, switching, splitting, coupling, and combining.
Generally, VOAs are designed to control signal power levels in optical networks, typically to reduce the power level in optical networks to a desired set point. A VOA may achieve the desired functionality of power reduction by diverting optic energy towards or away from a desired direction. A VOA used at an interconnection of optic fibers may have a desired coupling efficiency of anywhere from essentially zero to one hundred percent depending upon the required reduction of power level. In a special case when a desired coupling efficiency of a VOA is zero, it is known to operate as a shutter, meaning that none of the energy input to the VOA is output to the optical fiber.
VOAs are of two fundamental types, mechanical and non-mechanical. The mechanical VOAs may have moving parts such as stepper motors to adjust an optical filter to vary the attenuation. In non-mechanical VOAs, the mechanism employed to adjust the attenuation may be a magneto-optic effect that modifies a light waveguide (Other non-mechanical VOAs including crystal VOAs are also well known). The attenuation settings of non-mechanical VOAs are generally wavelength dependent. Mechanical VOAs on the other hand provide or adjust the optical attenuation in a manner that is relatively independent of wavelength. However, mechanical VOAs are known to have a number of problems, including instability, backlash, etc. Mechanical VOAs are less reliable as they tend to have increased susceptibility to shock and vibration. As mechanical resonances are long on the time scale of the network traffic, large blocks (megabytes) of data can be affected while a mechanical VOA recovers from a shock or vibration. On the other hand, backlash denotes inaccuracy in optical signal attenuation setting when adjusting the mechanical VOA device.
Ideally, the VOA should produce a continuously variable optical signal attenuation while introducing a normal or suitable insertion loss and while exhibiting a desired optical return loss. If the VOA causes excessive reflectance back toward the transmitter, the transmitter may become less stable, undermining the effectiveness of the VOA connection. It is generally desired that a VOA can be produced in a cost effective manner. Similarly, it is also desirable that a VOA has a small foot print and a straight-forward control mechanism.